This invention relates to blood pressure measurement apparatus, and more particularly to an improved cuff pressure controller for such apparatus.
Blood pressure measurement apparatus is known of the type wherein auscultatory (Korotkoff) sounds are detected as the air pressure is bled from an inflatable cuff, in order to determine the systolic and diastolic pressures and to display them. Such apparatus has been improved by substituting automatic inflation and deflation of the cuff for the conventional manual squeeze bulb and manual bleed valve. Examples of such blood pressure measurement devices and techniques are illustrated by U.S. Pat. No. 4,328,810--Hill et al. This patent illustrates the blood pressure measurement device with a controller for a pump to inflate the cuff and a solenoid valve to bleed the cuff under proportional control, using a constant frequency variable duty cycle arrangement. Hill employs a pulse train and varies the duty cycle to maintain a constant deflation rate by comparing the actual rate of change of pressure and the desired rate of change of pressure.
U.S. Pat. No. 4,313,445 issued Feb. 2, 1982 to Georgi similarly illustrates a computer controlled blood pressure measurement apparatus measuring Korotkoff sounds to determine blood pressure during controlled decrease of cuff pressure by automatic bleeding of air from the cuff under computer control.
U.S. Pat. No. 4,326,536 issued to Kitagwa et al. on Apr. 27, 1982 shows an automatic pumping, pressure bleeding and pressure dumping control for an auscultatory sphygmomanometer, wherein bleeding of pressure is regulated by a mechanical diaphragm-controlled slow exhaust valve, with solenoid coil to close the valve during an intermittent pumping cycle.
U.S. Pat. No. 4,295,471--Kaspari issued Oct. 20, 1981 discloses apparatus for automatically inflating and deflating a cuff, with a pressure transducer measuring cuff pressure, and suggests bleeding cuff pressure at a reduced rate during one part of the cycle and at a higher rate during other parts of the cycle.
U.S. Pat. No. 4,312,359--Olson issued Jan. 26, 1982 controls the cuff deflation by opening a pressure relief valve for short periods of time on an intermittent basis.
U.S. Pat. No. 3,905,354--Lichowsky issued Sept. 16, 1975 employs proportional valves to admit fluid under pressure to inflate the cuff and to bleed fluid pressure from the cuff, seeking to produce a substantially linear cuff pressure decrease in response to pressure rate-of-change feedback signals from a differentiating circuit.
British Application GB No. 2 087 238A discloses an automatic pumping and depressurizing device under control of a microprocessor, programmed to provide a signal which linearly decreases the pressure by opening and closing solenoid valves.
Lastly, U.S. Pat. No. 4,378,807 to Peterson et al. issued Apr. 5, 1983 shows an automatic cuff pressurizing and depressurizing system for a blood pressure measurement device of the type described, in which pressure lamp linearization circuits are employed. A decreasing pressure ramp reference signal is produced by discharging a capacitor through a constant current source to provide a negative-going ramp. A difference amplifier compares the reference voltage with actual voltage sent by a pressure transducer, and controls off-on valves with width modulated pulses. The modulator valve bleeding the cuff is therefore opened and closed for different periods of time, depending upon deviation in the cuff pressure from the generated reference ramp.
One of the disadvantages in the Peterson et al. patent with the use of off-on valves controlled using a variable duty cycle is that the off-on operation of the valves may create electrical noise through the power supply and acoustic noise generated by valve opening and closing. This noise may interfere with the Korotkoff sound detection. It would be desirable to have a cuff pressure bleed rate control device which provides a substantially linear pressure decrease with minimum electrical and acoustic noise generation to interfere with the readings.
It would also be desirable to provide an improved cuff pressure controller having a simplified circuit to generate substantially linear, decreasing, pressure reference signal. Such an improved cuff pressure controller circuit would include provision for initial inflation of the cuff, initialization of bleeding air from the cuff after reaching a desired pressure, controlled bleed rate during measurement, and termination of the deflation/measurement phase by dumping pressure from the cuff as quickly as possible.
Accordingly, one object of the present invention is to provide an improved cuff pressure controller for an auscultatory blood pressure measurement device.
Another object of the present invention is to provide an improved cuff pressure automatic controller for a microcomputer-controlled blood pressure measurement device, which controls inflation, bleed, and dumping of the cuff.
Still another object of the invention is to provide an improved circuit for controlling a bleed valve during depressurization of a cuff with minimum acoustic and electrical noise.